Some studies attribute up to 80% of all cancers can be attributed to environmental factors ("exogenous" cause rather than "endogenous cause"). This means they are not genetically triggered or an inevitable consequence of the ageing process and are therefore potentially preventable. Factors with which such environmental cancers are associated include: diet, tobacco, reproductive/sexual behaviour and exposure to pollutants.
Few biological agents are known to be associated with cancer in man (although, for example, the fluke infection, schistosomiasis, is known to cause bladder cancer). Such an association has been established for a number of physical and chemical agents. The term chemical carcinogen in its widely-accepted sense is used to indicate a substance that is known conclusively to induce or enhance the incidence of neoplasia.
There are 18 chemicals, groups of chemicals and industrial processes which are known to be carcinogenic in man, and a further 18 which re probably carcinogenic. Several hundred chemicals have been shown to be carcinogenic in experimental animals. All substances for which conclusive evidence of carcinogenicity exists in man have also been proved to be carcinogenic in animals, with the possible exception of arsenic. Whether an animal carcinogen causes cancer in man depends on many factors, including inter-species biological differences, and the duration and extent of exposure and the measures taken to reduce these.
Changes in cancer incidence patterns among immigrants is revealing - the patterns gradually change from those prevalent in the country of origin to approximately those of the host country (as happened with Japanese immigrants to California and Hawaii). The recent increase in malignant melanoma is the direct result of changes in clothing habits and the fashion of being tanned. Other increases result from medical practices - the increase in thyroid cancer as a consequence of childhood exposure of the head and neck to X-rays, and the increase in endometrial cancer as a consequence of the use of menopausal oestrogens. The most important environmental factor in carcinogenesis is tobacco smoking, which alone accounts for 25-35% of all cancer deaths in men and 5-10% in women.
In 1998, the California Air Resources Board identified diesel exhaust as a "Toxic Air Contaminant" based on a review of animal and epidemiological studies, which strongly suggest a causal relationship between occupational diesel exhaust exposure and lung cancer. Already in 1989, the International Agency for Research on Cancer (IARC) had concluded that diesel engine exhaust is "probably carcinogenic to humans" (Group 2A), while gasoline engine exhaust was classified as "possibly carcinogenic to humans" (Group 2B).